The parietal eye of lizards is a photosensitive organ located in the dorsal midline of the animal. Its photoreceptors synapse directly onto ganglion cells, but afferent signal flow is additionally modified by centrifugal input. It is proposed to define the receptive fields and light-evoked responses of both classes of intrinsic neurons through single cell recordings. The effects of centrifugal fiber activation upon afferent activity will be tested through electrical and chemical stimulation of the centrifugal path. Morphological studies employing a variety of light-microscopic and electron microscopic techniques will be utilized to characterize the neuronal geometry, synapses and central connections of the parietal eye.